Aptitude - Odd Man Out and Series - Discussion

Find out the wrong number in the given sequence of numbers.

5. 

1, 2, 6, 15, 31, 56, 91

[A]. 31
[B]. 91
[C]. 56
[D]. 15

Answer: Option B

Explanation:

1, 1 + 12 = 2, 2 + 22 = 6, 6 + 32 = 15, 15 + 42 = 31, 31 + 52 = 56, 56 + 62 = 92

Last number of given series must be 92 not 91.


Harsh said: (Apr 7, 2011)  
Only the sum of first and last number is divisible by 5.

Dhivya said: (Nov 14, 2011)  
Can someone explain this clearly ?

Kumar said: (Dec 25, 2011)  
2-1,6-2,15-6,31-15,56-31,91-56
1,4,9,16,25,45
last number is clearly wrong because those are perfect squares

Sonutalwani said: (Jun 29, 2012)  
1 + (1*1) = 2.
2 + (2*2) = 6.
6 + (3*3) = 15.
15 + (4*4) = 31.
31 + (5*5) = 56.

56 + (6*6) = 92 (91 is wrong number).

Ravi Agola said: (Aug 25, 2012)  
Actual answer by easy method.

1 is odd then two are even 2, 6 then two are odd 15, 31 then two are even 56, 91 but here 91 is odd so answer. Is 91 which we same get with previous method.

Sikkanthar said: (Aug 28, 2013)  
In this problem they are adding the result with square value of the natural numbers.
I mean.

1,
1(result) + (1*1) = 2.
2(result) + (2*2) = 6.
6(result) + (3*3) = 15.
15(result) + (4*4) = 31.
31(result) + (5*5) = 56.
56(result) + (6*6) = 92.

Now we are getting 92 not 91.. So 91 is the wrong number of the given.

Rajesh said: (Sep 12, 2013)  
@Kumar.

Your concept is OK. But same time you took the wrong concept.

First minus 1 by 2.

Then minus 6 by 2 how is it correct? OK now you understand.

But your thing is good.

Umang said: (Oct 6, 2013)  
2-1 = 1*1
6-2 = 2*2
15-6 = 3*3
31-15 = 4*4
64-31 = 5*5

But 91-56 = 6*6 is not happening.

So 91 is odd no.

Sam said: (Aug 17, 2014)  
10 teams participating in national football league. If each team plays the other team two times, the total number of games the league will play is.

Beda said: (Nov 26, 2014)  
In question 31 is the only prime number. So why the answer is not 31?

Saran said: (Jan 2, 2015)  
Answer is 91 but calculate answer is 92. So it is a wrong no.

Riya said: (Jul 16, 2015)  
According to me 31 should be the answer because its the only prime no in the question?

Moni said: (Jul 28, 2015)  
What is the answer for sum?

Debasish said: (Aug 31, 2015)  
6-2 = 4 = 2*2.
15-6 = 9 = 3*3.
31-15 = 16 = 4*4.
56-31 = 25 = 5*5.
91-56 = 35! = 6*6.

Sreeram said: (Feb 2, 2016)  
Why not 31? It's the only prime number.

Manish said: (Jul 5, 2016)  
If we see prime numbers. Then 31 will be the answer. So which is the correct answer?

Arthy said: (Oct 23, 2016)  
2 - 1 = 1(odd),
6 - 2 = 4(even),
15 - 6 = 9(odd),
31 - 15 = 16(even),
56 - 31= 25(odd),
But 91 - 56 = 45 (odd) so that 91 is the wrong number here.

Fathima said: (Jan 28, 2017)  
31 is the only prime number.

Why not the answer is 31? Some one explain.

Aditya said: (Mar 18, 2017)  
2, 5, 15, 131, 530, 13257 find out wrong number of the series.

Jatin Dwivedi said: (Nov 20, 2017)  
Answer should be 31.

Utkarsh Kumar said: (Jul 22, 2018)  
2 is also a prime number. So 31 cannot be the answer even if we consider this question in terms of prime numbers.

This question is solved on the basis of perfect squares. Subtract each adjacent number and you will get a perfect square, except:

91-56 = 35.

35 is not a perfect square, so we make it 36 by adding 1 to 91. Thus the final series will be as follows:

1, 2, 6, 15, 31, 56, 92
where subtracting any two adjacent numbers will give a perfect square.

I hope this helps.

Post your comments here:

Name *:

Email   : (optional)

» Your comments will be displayed only after manual approval.